1. Field of the Invention
The present invention reltes to a dielectric film, a laminated film using the same, and a process for preparing the same.
2. Brief Description of the Prior Art
A conventional dielectric film having a high dielectric constant is one formed from a mixture of a ceramic dielectric with a high dielectric constant and a polymer. Where an amount of the high dielectric ceramics is increased, processability of the dielectric film resulting therefrom is impaired. Where the amount thereof is rendered small, the dielectric constant of the resulting dielectric film is decreased.
As a conventional dielectric body having a high dielectric constant has a high dielectric dissipation factor, tan .delta., there is known yet no dielectric film having a combination of properties such as high dielectric constant, low AC loss, good processability and uniform composition.
A conventional dielectric film employing a polymer as a matrix material presents various problems. For example, it is known that a capacitor or a capacitive switch which results from a film of a thermoplastic resin such as polyolefin, thermoplastic polyester, polyamide, polycarbonate, polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene or polyvinyl chloride possesses an electrostatic capacity that is in proportion to a dielectric constant of the film and in inverse proportion to a thickness of the film. Although it is preferred that the thickness of the film is as thin as possible, there is a limitation of rendering the film thinner. For example, a thermoplastic polyester film having the thickness of 2 microns and a high Young's modulus is commercially available; however, it is very difficult to form a film having a thickness of less than 3 microns from the other resins on an industrial scale. If a film which is formed from a material having a Young's modulus far smaller than that of a film of polyester such as polyvinylidene fluoride or polypropylene is wound on itself in roll, a stress resulting from winding without causing wrinkles may sometimes exceed the critical stress of the elastic limit of the film. This will encounter the difficulty with respect to the preparation of a capacitor from such film roll. Such conventional film will also give rise to the difficulty in increasing an electrostatic capacity of the capacitor so that the resulting capacitor cannot be rendered thinner.
Japanese Patent Publication No. 2,044/1980 discloses a composition comprising polyvinylidene fluoride and carbon black. This composition permits a large variation in dielectric constant with a slight change in an amount of carbon black. It has the tendency that carbon black decomposes upon application of a high shear during milling, resulting in a variation in dielectric constant even when carbon black is mixed in identical amounts. With such composition, it is difficult to form films having a uniform thickness on an industrial scale. Where the amount of carbon black is rendered too large, such composition has the tendency that the dissipation factor, tan .delta., becomes extremely low.